Optical pickup device

ABSTRACT

An optical pickup device for condensing the light from a light source on a signal recording surface by an objective lens and for detecting the return light from the signal recording surface by a light detection unit is disclosed. The optical pickup device is capable of recording and/or reproducing at least two different sorts of optical recording media. The light beam radiated from a light source is condensed by an objective lens. An aperture varying unit varies the size of an aperture of the objective lens depending upon the sorts of the optical recording media. The return light from the optical recording medium is detected by a light detection unit. The size of the aperture of the objective lens for condensing the light from the light source is varied depending upon the difference in substrate thickness of the different sorts of the optical recording media, thus assuring high-quality recording/reproduction for two or more different sorts of optical recording media.

BACKGROUND OF THE INVENTION

This invention relates to an optical pickup device for condensing thelight from a light source on a signal recording surface by an objectivelens and for detecting the return light from the signal recordingsurface by light detection means.

For recording/reproducing optical or magneto-optical signals on or froman optical recording medium, such as an optical disc or amagneto-optical disc, an optical pickup device, which radiates a laserlight beam to the optical recording medium and detects a return lightbeam therefrom, is employed.

The general construction of the optical pickup device is shown forexample in JP Patent Kokai Publication JP-A-54-39101 (1979), shownherein in FIG. 1. This conventional optical pickup device is soconstructed that a laser light beam from a laser diode 71 is condensedby an objective lens 71 73on a signal recording surface of an opticaldisc 74 and a return light therefrom is detected by a photodetector 75to produce a servo signal and an RF signal. Thus the optical pickupdevice is of a simplified design of a finite multiplication factor whichhas omitted the collimator lens. In particular, the optical pickupdevice includes a beam splitter 72 disposed obliquely between theobjective lens 73 and the photodetector 75.

With the present optical pickup device, part of the laser light radiatedfrom the laser diode 71 is reflected by the surface of the beam splitter72, obliquely arranged between the objective lens 73 and thephotodetector 75, so as to be directed to the objective lens 73. Theobjective lens 73 condenses the laser light to radiate it on the signalrecording surface of the optical disc 74.

The return light reflected by the signal recording surface on theoptical disc 74 again reaches the beam splitter 72 via the objectivelens 73. The beam splitter 72 permits part of the return light to bepassed therethrough to fall on the photodetector 75.

With recent increase in the volume of information, the optical recordingmedium, as the package medium of the music or picture information, suchas a recording device for a computer, compact disc or a video disc, havebecome promulgated extensively, while the tendency is also towardsrecording with higher recording density.

Among the methods for achieving such high density recording, it may alsobe contemplated to increase the numerical aperture (NA) of the objectivelens so as to be larger than that used in an optical pickup deviceemployed in, e.g., a conventional compact disc player. If NA isincreased, the beam spot formed on the compact disc is reduced in size,thus leading to improved resolution and higher recording density.However, if NA is increased, the tolerance for disc tilt is diminished.

For example, the optical disc reproducing device reproduces signalsrecorded on the reflective surface, that is the signal recordingsurface, via a transparent substrate having a thickness on the order of1.2 mm. Thus, should the disc be tilted with respect to the optical axisof the objective lens, the third-order coma aberration, generated inproportion to approximately a third power of the numerical aperture NAand approximately a first power of a skew θ, becomes dominant. Thegenerated wavefront surface W is given by

W(x, y)=W ₃₁ x(x ² +y ²)  (1)

On the other hand, the Seidel's third-order coma aberration coefficientW₃₁ becomes $\begin{matrix}{W_{31} = {\frac{n^{2} - 1}{2n^{2}}{{t\theta NA}^{3}/\lambda}}} & (2)\end{matrix}$

because θ is sufficiently small. The unit is standardized by thewavelength λ.

In the above equations, t denotes a thickness of the disc substrate, ndenotes a refractive index of the disc substrate, θ denotes the quantityof disc skew, and NA is the numerical aperture NA of the objective lens.

In a system having the numerical aperture NA of, e.g., 0.6, which is asmuch as 1.33 times the numerical aperture NA of 0.45 of an objectivelens employed in an optical pickup device of a conventional opticalpickup device, a coma aberration of about 3.5 times as much as thatproduced with a conventional system is produced for the disc skew whichis of the same magnitude as that of the conventional compact disc.

For example, if a disc having a disc skew as large as ±0.5 to 1°, suchas a disc having a polycarbonate substrate produced inexpensively inlarge quantities, is employed, the spots spot formed on the disc becomesnon-symmetrical due to such wavefront distortion, thus increasinginter-symbol interference, such that waveform distortion in thereproduced signal becomes significant and hence the signal cannot beextracted sufficiently.

Thus it may be contemplated to reduce the thickness t of the discsubstrate from, e.g., 1.2 mm to, e.g., 0.6 mm, that is to a one-halfvalue, for reducing the third-order coma aberration coefficient W₃₁ to aone-half value.

Meanwhile, if the disc substrate is of a small thickness, as describedabove, and a pre-existing optical recording medium, such as a write-onceoptical disc, phase-change optical disc or a magneto-optical disc, isreproduced using an optimized objective lens, RF signals of excellentsignal quality cannot be reproduced due to the difference in substratethicknesses.

It is known in general that the amount of fourth-order sphericalaberration, standardized with the wavelength generated with parallelflat plates in the converging light beam, is proportional to the fourthpower of the numerical aperture NA of the objective lens andproportional to a reciprocal of the wavelength. On the other hand, thewavefront W is given by

W(x, y)=W ₄₀(x ² +y ²)²  (3)

The Seidel's fourth-order spherical aberration coefficient W₄₀ is givenby $\begin{matrix}{W_{40} = {\frac{n^{2} - 1}{n^{3}}\frac{t}{8}{{NA}^{4}/\lambda}}} & (4)\end{matrix}$

where t and n denote the thickness and the refractive index of thesubstrate, respectively.

Thus, if a compact disc having a substrate thickness t=1.2 mm isreproduced using an objective lens of a numerical aperture of 0.6,optimized for the substrate thickness t=0.6 mm, the amount of thegenerated spherical aberration reaches a larger value of 3.6 μm in termsof the Seidel's fourth-order aberration coefficient W₄₀. The root meansquare sum of the aberration of the optical system in this case is 0.268rmsμm. For comparison, root mean square sum values for the wavelength λof 0.532 μm and for the wavelength λ of 0.68 μm are 0.5 rmsλ and 0.4rmsλ, respectively.

It is generally required of the optical disc that the root mean squaresum of the aberration of a given optical system be not more than 0.07rmsλ in terms of the Marechal criterion. Thus, such a system in whichthe root mean square sum ascribable only to the disc substrate amountsto as much as 0.4 rmsλ to 0.5 rmsλ is not acceptable.

That is, if two or more sorts of the optical disc having different discsubstrate thicknesses are to be reproduced, playback signals of highsignal quality cannot be obtained with a disc having a substratethickness for which the objective lens is not optimized.

OBJECT AND SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide an opticalpickup device whereby high-quality recording/reproduction may beachieved even with two or more different optical recording media.

With the optical pickup device according to the present invention, thesize of the aperture of the objective lens for condensing the light froma light source is varied depending upon the sorts of the optical discsin order to enable recording and/or reproduction of at least twodifferent sorts of optical recording media.

With the above optical recording medium, it is possible for the aperturevarying means to set a numerical aperture NA of the objective lens withrespect to the optical recording medium having a substrate thickness of0.6 mm to 0.6 as well as to set a numerical aperture NA_(o) of theobjective lens with respect to the optical recording medium having asubstrate thickness of 1.2 mm to not more than (0.45/0.78)×λ_(o), whereλ_(o) denotes a numerical value equal to the wavelength of lightradiated by the light source.

The aperture varying means for varying the size of the aperture of theobjective lens depending upon the sorts of the optical recording mediummay be arranged between the objective lens and the light source orbetween the objective lens and the optical recording medium.Alternatively, the aperture varying means may be arranged on the surfaceof or within the inside of the objective lens.

The aperture varying means may also vary the aperture of the objectivelens depending upon the substrate thickness of at least two differentsorts of the optical recording media. The aperture varying means mayalso vary the aperture size by rotating respective blades of a lightstop mechanism having the plural blades.

The aperture varying means may also vary the aperture size using aliquid crystal shutter or by moving of or rotating a plate having pluralopenings for changing over the openings.

With the optical pickup device according to the present invention, sincethe size of the aperture of the objective lens for condensing the lightfrom a light source is varied depending upon the difference in substratethickness of the different sorts of the optical recording media byaperture varying means, high-quality recording/reproduction may berealized for disc-shaped recording media of two or more different sorts.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing a conventional optical pickup device.

FIG. 2 is a schematic view of an optical pickup device according to afirst embodiment of the present invention.

FIG. 3 illustrates reproduction of information signals from a dischaving a disc substrate thickness of 0.6 mm by the optical pickup deviceshown in FIG. 2.

FIG. 4 illustrates reproduction of information signals from a dischaving a disc substrate thickness of 1.2 mm by the optical pickup deviceshown in FIG. 2.

FIGS. 5A and 5B illustrate an aperture varying mechanism for varying thesize of the aperture by rotating plural blades.

FIGS. 6A and 6B illustrate on-axis wavefront aberration in case ofreproduction of information signals from a disc having a disc substratethickness of 0.6 mm by the optical pickup device shown in FIG. 2.

FIGS. 7A and 7B illustrate on-axis wavefront aberration in case ofreproduction of information signals from a disc having a disc substratethickness of 1.2 mm by the optical pickup device shown in FIG. 2.

FIG. 8 is a graph showing the relation between the spherical aberration(rmsλ) and the numerical aperture (NA) in case of radiation of a laserlight beam with the laser light wavelength λ of 0.635 μm to a coverglass with a substrate thickness of 1.2 mm using an objective lensoptimized for the cover glass with a substrate thickness of 0.6 mm.

FIG. 9 is a graph showing changes in the amount of jitter for each ofvalues of the numerical aperture of 0.30, 0.33 and 0.36, with the discskew being 0°.

FIG. 10 is a graph showing changes in the amount of jitter for each ofvalues of the numerical aperture of 0.30, 0.33 and 0.36, with thetangential disc skew being 1°.

FIG. 11 is a graph showing changes in the amount of jitter for thenumerical aperture NA=0.6 for the optical disc 5 with the substratethickness t₁=0.6 mm.

FIG. 12 is a graph showing changes in the amount of jitter for thenumerical aperture NA₀=0.33 for the optical disc 7 with the substratethickness t₂=1.2 mm.

FIG. 13 is a schematic view illustrating light stop adjustment of theaperture varying mechanism for the numerical aperture of the objectivelens 4 of 0.33.

FIG. 14 is a schematic view of an optical pickup device according to asecond embodiment of the present invention.

FIG. 15 is a schematic view of an optical pickup device according to athird embodiment of the present invention.

FIG. 16 is a schematic view of an optical pickup device according to afourth embodiment of the present invention.

FIG. 17 is a schematic view of an optical pickup device according to afifth embodiment of the present invention.

FIG. 18 is a schematic view of an optical pickup device according to asixth embodiment of the present invention.

FIG. 19 illustrates the operation of reproducing an optical disc with asubstrate thickness of 1.2 mm with an optical pickup device shown inFIG. 18.

FIGS. 20A and 20B illustrate on-axis wavefront aberration in case ofreproduction of information signals from a disc having a disc substratethickness of 0.6 mm by the optical pickup device shown in FIG. 18.

FIGS. 21A and 21B illustrate on-axis wavefront aberration in case ofreproduction of information signals from a disc having a disc substratethickness of 1.2 mm by the optical pickup device shown in FIG. 18.

FIGS. 22A, 22B illustrate the construction of the aperture varyingmechanism configured for varying the aperture size using a liquidcrystal shutter.

FIG. 23 illustrates the construction of the aperture varying mechanismconfigured for varying the aperture size by moving a plate provided withplural apertures.

FIG. 24 illustrates the construction of the aperture varying mechanismconfigured for varying the aperture size by moving a disc provided withplural apertures.

FIG. 25 is a section view of an optical pickup device having an aperturevarying mechanism positioned between an objective lens and a opticaldisk recording medium.

FIG. 26 is a section view of an optical pickup device having an aperturevarying mechanism arranged within the objective lens.

FIG. 27 is a section view of an optical pickup device having an aperturevarying mechanism arranged on a surface of the objective lens.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to the drawings, preferred embodiments of the optical pickupdevice according to the present invention will be explained in detail.

The optical pickup device of the first embodiment of the presentinvention is explained. The present first embodiment is directed to anoptical pickup device 8 configured for reproducing information signalsfrom an optical disc 5 with a substrate thickness of 0.6 mm and anoptical disc 7 with a substrate thickness of 1.2 mm, as shown in FIG. 2.The optical pickup device includes a laser diode 1 for radiating theplayback laser light to the optical discs 5 and 7 and an objective lens4 for condensing the playback laser light from the laser light 1 on thesignal recording surfaces (information surfaces) of the optical discs 5and 7. The optical pickup device also includes an aperture varyingmechanism 3 for setting the numerical aperture NA of the objective lens4 for the optical disc 5 with the substrate thickness of 0.6 mm to 0.6and that for the optical disc 7 with the substrate thickness of 1.2 mmto 0.4, and a photodetector 6 for detecting the return light from theoptical discs 5, 7. The aperture varying mechanism 3 is arranged betweenthe objective lens 4 and the laser diode 1.

The optical pickup device 8 causes part of the laser light radiated fromthe laser diode 1 to be reflected by a beam splitter 2 so as to becondensed on the signal recording surfaces of the optical discs 5 and 7via the aperture varying mechanism 3 and the objective lens 4. Thereturn light from the signal recording surfaces of the optical discs 5and 7 is incident via the aperture varying mechanism 3 on the beamsplitter 2. Part of the return light is transmitted through the beamsplitter 2 so as to fall on the photodetector 6. The photodetector 6converts the volume of the return light into electrical signals. Thus,with the present first embodiment, servo signals and RF signals areproduced. Meanwhile, the optical pickup device 8 of the present firstembodiment is of a simplified construction of a finite multiplicationfactor not having a collimator lens.

The outgoing laser light from the laser diode 1, reflected by the beamsplitter 2, is incident on the objective lens 4 via the aperture varyingmechanism 3. The aperture varying mechanism 3, arranged between theobjective lens 4 and the laser diode 1, represents aperture varyingmeans whereby the aperture size may be changed depending upon thesubstrate thickness of the optical disc 5 or 7. In effect, the aperturevarying mechanism 3 varies the numerical aperture of the objective lens4.

It is possible with the present optical pickup device 8 to judge thedifference in the substrate thickness of 0.6 mm and 1.2 mm of theoptical discs 5 and 7 by substrate thickness detection means 6a, asshown in FIG. 2. Among substrate thickness detection means, there aresuch means for mechanically, optically or magnetically detecting whetheror not the optical disc is housed within the cartridge main bodyportion, means for detecting the information on the substratethicknesses, recorded on the optical disc by, e.g., bar codes, usinglight sources, such as laser diodes, LEDs or lamps, and means (e.g., asdepicted in FIG. 2) for detecting the difference in substrate thicknessusing RF or servo signals detected by the photodetector 6.

The operating principle of the optical pickup device 8 is explained byreferring to FIGS. 3 and 4.

For reproducing information signals from the optical disc 5 with asubstrate thickness t₁ of 0.6 mm, the light stop of the aperture varyingmechanism 3 and the aperture diameter of the objective lens 4 are set to4.32 mm and to 4.32, respectively, for setting the numerical aperture ofthe objective lens 4 to 0.6. On the other hand, for reproducinginformation signals from the optical disc 7 with a substrate thicknesst₂ of 1.2 mm, the light stop of the aperture varying mechanism 3 and theaperture diameter of the objective lens 4 are set to 2.88 mm and to2.88, respectively, for setting the numerical aperture of the objectivelens 4 to 0.4. At this time, the laser light having the wavelength of680 mm is radiated from the laser diode 1 so that the focal length willbe 3.6 mm.

The aperture varying mechanism 3 is such a mechanism in which pluralblades 21a, 21b, 21c and 21d of the light stop mechanism are rotated forvarying the light stop for varying the size of the aperture 25, as shownin FIGS. 5A and 5B. For reducing the aperture 25, a rotary unit 20 isturned in a direction of arrow R so that pins 22a, 22b, 22c and 22d willbe moved in the direction of arrow R, as shown in FIG. 5A. The blades21a, 21b, 21c and 21d, whose elongated openings 23a, 23b, 23c and 23dare engaged by the pins 22a, 22b, 22c and 22d, are supported at one endsthereof by supporting portions 24a, 24b, 24c and 24d, so that the blades21a to 21d have opposite ends thereof directed towards the innerperiphery of the aperture varying mechanism 3. Thus the aperture 25 isreduced in diameter.

For enlarging the aperture 25, the rotary unit 20 is rotated in adirection of arrow L in FIG. 5B. The blades 21a, 21b, 21c and 21d havetheir opposite ends directed towards the outer periphery of the aperturevarying mechanism 3. Thus the aperture 25 is enlarged in diameter.

Evaluation of the operation of the optical pickup device 8 is done withthe Marechal criterion of 0.07 rmsλ as a reference.

In general, the non-spherical shape z of the objective is given by$\begin{matrix}{z = {\frac{({CURV})h^{2}}{1 + \sqrt{1 - {\left( {1 + k} \right)({CURV})^{2}h^{2}}}} + {Ah}^{2} + {Bh}^{6} + {Ch}^{8} + {Dh}^{10}}} & (5)\end{matrix}$

where CURV denotes the radius of curvature of the inscribednon-spherical surface, k denotes the constant of a conus, A denotes thefourth-order non-spherical coefficient, B denotes the sixth-ordernon-spherical coefficient, C denotes the eighth-order non-sphericalcoefficient and D denotes the tenth-order non-spherical coefficient.

The objective lens 4 has the following lens dimensions. The lensdimensions are given in the order of the radius of curvature, spacingand the refractive index according to respective planes. The radius ofcurvature and the spacing of the object plane are both infinite (∞).

The radius of curvature, spacing and the refractive index of the firstplane are 2.40336, 2.460000 and 1.585352, respectively. Thenon-spherical coefficients of the first plane shown in the equation (5)are k=−0.1116645, A=−0.175589E−03, B=0.371319E−04, C=0.000000E+00 andD=0.000000E+00.

The radius of curvature and the spacing of the second plane are−0.10.64036 and 2.155501, respectively. The non-spherical coefficientsof the second plane shown in the equation (5) are k=−0.17298E09,A=0.213134E−01, B=0.128092E+00, C=0.000000E+00 and D=0.000000E00.

As to the lens design specifications, the numerical aperture NA is0.60000, the wavelength is 680.00 nm, the focal lengths is 3.6, and animage height is 0.05000 mm, for the substrate thickness t₁=0.6 mm,whereas, for the substrate thickness t₂=1.2 mm, the numerical apertureNA is 0.4, the wavelength is 680.00 nm, the focal length is 3.6, and animage height is 0.05000 mm.

The on-axis wavefront aberration is shown in FIGS. 6A, 6B, 7A and 7B.FIG. 6A shows changes in the aberration in the meridinal image surfacewith respect to changes in the diameter of the pupil of the objectivelens 4 for the disc substrate thickness t₁=0.6 mm and the numericalaperture of the objective lens of 0.6. FIG. 6B shows changes in theaberration in the sagittal image surface with respect to changes indiameter of the pupil of the objective lens 4. FIG. 7A shows changes inthe aberration in the meridinal image surface with respect to changes inthe diameter of the pupil of the objective lens 4 for the disc substratethickness t₂=1.2 mm and the numerical aperture of the objective lens of0.4. FIG. 7B shows changes in the aberration in the sagittal imagesurface with respect to changes in diameter of the pupil of theobjective lens 4. The on-axis aberration of the optical system is 0.029rmsλ and 0.043 rmsλ for the disc substrate thickness t₁=0.6 mm and thenumerical aperture of the objective lens of 0.6 and for the discsubstrate thickness t₂=1.2 mm and the numerical aperture of theobjective lens of 0.4, respectively. In both of these cases, theMarechal criterion was not higher than 0.07 rmsλ. Thus, with the firstillustrative example, high-quality reproduction may be achieved with thetwo sorts of the optical disc 5 of the disc substrate thickness t₁=0.6mm and the optical disc 7 of the disc substrate thickness t₂=1.2 mm.

The aperture varying mechanism 3 of the optical pickup device 8 may beso designed that the numerical aperture NA of the objective lens 4 forthe optical disc 5 with the substrate thickness of 0.6 mm is 0.6 and thenumerical aperture NA_(o) of the objective lens 4 for the optical disc 5with the substrate thickness of 1.2 mm is not more than(0.45/0.78)×λ_(o), where λ_(o) is a numerical value equal to thewavelength of the laser light radiated by the laser diode 1 in μm.

The value of 0.45 is the commonplace value of the numerical aperture ofthe objective lens employed for recording/reproduction of the opticalrecording medium having the substrate thickness of 1.2 mm, such as acompact disc, while the value of 0.78 is the value for the wavelength oflight from the light source employed on this occasion.

Since the aperture varying mechanism 3 is such a mechanism which givesthe numerical aperture NA_(o) of the objective lens 4 equal to not morethan (0.45/0.78)×λ_(o), where λ_(o) is a numerical value equal to thewavelength of the laser light radiated by the laser diode 1 in μm, forthe optical disc 5 with the substrate thickness of 1.2 mm, as describedabove. Thus, if the wavelength λ of the laser light radiated from thelaser diode 1 is, e.g., 0.635 μm, the numerical aperture NA_(o) is notmore than 0.366.

The relation between the spherical aberration (rmsλ) and the numericalaperture NA on radiating the laser light on a cover glass having thesubstrate thickness of 1.2 mm, using an objective lens optimized for thecover glass with the substrate thickness of 0.6 mm, with the laser lightwavelength λ being 0.635 μm, is shown in FIG. 8. The sphericalaberration is increased at this time approximately as a fourth power ofthe numerical aperture.

The commonplace cut-off frequency of the reproducing optical system foran optical disc, such as a compact disc, is 1153/mm. The numeralaperture for obtaining this cut-off frequency for the light sourcewavelength of 0.635 μm is 0.366. By simulation, values of the sphericalaberration shown in Table 1:

7λ = 635 nm λ = 780 nm NA 0.30 0.33 0.36 0.45 Skew° 0.027 rmsλ 0.040rmsλ 0.056 rmsλ 0 rmsλ Skew t1° 0.029 rmsλ 0.042 rmsλ 0.059 rmsλ 0.058rmsλ Skew r1° 0.029 rmsλ 0.042 rmsλ 0.059 rmsλ 0.058 rmsλ

were obtained for the case of reproducing the optical disc with thevalues of the numerical aperture smaller than the above numerical value,for example, 0.30, 0.33 and 0.36. For each of these numerical apertures,three cases of skews, that is skew of 0°, tangential skew of 1°(specified t1°) and radial skew of 1′ 1° (specified as r° r1°) wereassumed. For comparison, Table 1 shows respective values of thespherical aberration for the above three skew values of the objectivelens with the numerical aperture for the laser light with the lightsource wavelength of 780 μm. In Table 1, the same values of thespherical aberration were achieved before and after waveform evaluationof the eye diagram obtained by simulation.

That is, it is seen from Table 1 that, if the value of the numericalaperture is not more than 0.360, the respective values of the sphericalaberration are not more than the Marechal criterion of 0.07 rmsλ, as inthe case of the three values of the skew of the objective lens with theNA of 0.45 for the laser light of the light source wavelength of 780 nm.

FIG. 9 shows changes in jitter for each value of the numeral aperturesof 0.30, 0.33 and 0.36 for the disc skew of 0°. In FIG. 9, a solid lineand broken line denote changes proper to jitter before and afterwaveform evaluation, respectively.

FIG. 10 shows changes in jitter, having a high contribution factor asperturbation, for respective values of the numerical aperture of 0.30,0.33 and 0.36 for the tangential disc skew of 1°. In FIG. 10, a solidline and a broken line denote changes proper to jitter before and afterwaveform evaluation, respectively.

Thus it is seen from FIGS. 9 and 10 that the jitter becomes minimalbefore and after waveform evaluation for the numerical aperture of 0.33,and that, if the values of the aberration shown in Table 1 are takeninto account, the numerical aperture of 0.33 is most preferred forreproducing the optical disc with the substrate thickness of 1.2 mm.

This holds for any wavelength of the light source, including the lightsource wavelength of 0.635 μm. That is, the optical disc with thesubstrate thickness of 1.2 mm may be reproduced satisfactorily bysetting the numerical aperture NA_(o) of the objective lens so as to benot more than (0.45/0.78)×λ_(o).

Thus, if the wavelength λ is 0.65 μm and 0.68 μm, the numerical apertureNA_(o) of not more than 0.375 and not more than 0.392 suffices,respectively. The optimum value of the numerical aperture may besearched after finding the spherical aberration and the jitter asdescribed above.

FIGS. 11 and 12 show the on-axis wavefront aberration of the objectivelens 4 of the optical pickup device 8 for the light source wavelength of0.635 μm. FIGS. 11 and 12 show the aberration of the objective lens 4with the numerical aperture NA of 0.6 for the optical disc 5 with thesubstrate thickness t₁=0.6 mm and aberration of the objective lens 4with the numerical aperture NA_(o) of 0.33 for the optical disc 7 withthe substrate thickness t₂=1.2 mm, respectively.

For the optical disc 5 with the substrate thickness t₁=0.6 mm, theaberration of the optical system becomes 0.001 rmsλ for the numeralaperture NA of the objective lens 4 of 0.6, whereas, for the opticaldisc 5 with the substrate thickness t₂=1.2 mm, the aberration of theoptical system becomes 0.042 rmsλ for the numerical aperture NA_(o) ofthe objective lens 4 of 0.33. In both cases, the Marechal criterion wasnot more than 0.07 rmsλ with sufficient allowance.

Thus it is seen that the optical pickup device 8 having the aperturevarying mechanism 3 which gives the numerical aperture NA_(o) of theobjective lens 4 of not more than (0.45/0.78)×λ_(o) for the optical disc7 with the substrate thickness t₂=1.2 mm is capable of satisfactorilyreproducing the optical disc with the substrate thickness t₁=0.6 mm andthe optical disc with the substrate thickness t₂=1.2 mm.

If the optical pickup device 8 is to reproduce information signals fromthe optical disc 5 having the substrate thickness t₁ of 0.6 mm, thelight stop of the aperture varying mechanism 3 is adjusted for settingthe numerical aperture of the objective lens 4 to 0.6 mm by setting theaperture diameter of the objective lens 4 to 4.32 mm, as shown in FIG.3. If the optical pickup device 8 is to reproduce information signalsfrom the optical disc 7 having the substrate thickness t₂ of 1.2 mm, thelight stop of the aperture varying mechanism 3 is adjusted for settingthe aperture diameter of the objective lens 4 to 2.38 mm, for therebysetting the numerical aperture of the objective lens 4 to 0.33, as shownin FIG. 13. The objective lens 4 radiates the laser light from the laserdiode 1 with the wavelength of 635 nm on the optical disc with the focallength of 3.6 mm.

The second embodiment is explained by referring to FIG. 14. Similarly tothe first embodiment for the optical pickup device 8, the present secondembodiment is directed to an optical pickup device 30 configured forreproducing information signals from an optical disc 5 with a substratethickness of 0.6 mm and an optical disc 7 with a substrate thickness of1.2 mm.

That is, the optical pickup device includes a laser diode 1 forradiating the playback laser light to the optical discs 5 and 7 and anobjective lens 4 for condensing the playback laser light from the laserlight 1 on the signal recording surfaces (information surfaces) of theoptical discs 5 and 7. The optical pickup device also includes anaperture varying mechanism 3 for setting the numerical aperture NA ofthe objective lens 4 for the optical disc 5 with the substrate thicknessof 0.6 mm and the numerical aperture NA_(o) of the objective lens 4 forthe optical disc 7 with the substrate thickness of 1.2 mm to 0.6 and to(0.45/0.78)×λ_(o), respectively, where λ_(o) denotes a numerical valueequal to the wavelength of the laser light radiated from the laser diode31 in μm, and a photodetector 6 for detecting the return light from theoptical discs 5, 7. The aperture varying mechanism 3 is arranged betweenthe laser diode 1 and the beam splitter 2.

The optical pickup device 30 causes part of the laser light radiatedfrom the laser diode 1 and transmitted through the aperture varyingmechanism 3 to be reflected by the beam splitter 2 so as to be condensedon the signal recording surfaces of the optical discs 5 and 7 via theobjective lens 4. The return light from the signal recording surfaces ofthe optical discs 5 and 7 is incident via the aperture varying mechanism3 on the beam splitter 2. Part of the return light is transmittedthrough the beam splitter 2 so as to fall on the photodetector 6. Thephotodetector 6 converts the volume of the return light into electricalsignals. Thus, with the present first embodiment, servo signals and RFsignals are produced. Meanwhile, the optical pickup device 8 of thepresent first embodiment is of a simplified construction of an infinitemultiplication factor not having a collimator lens.

The aperture varying mechanism 3 arranged between the laser diode 1 andthe beam splitter 2 is such an aperture varying means capable ofchanging the size of the aperture depending upon the substrate thicknessof the optical discs 5 and 7 and consequently changing the numeralaperture of the objective lens 4.

Similarly to the optical pickup device of the first embodiment, theoptical pickup device 30 is capable of discriminating the difference inthe substrate thickness of the optical disc 5 with the substratethickness of 0.6 mm and the optical disc 7 with the substrate thicknessof 1.2 mm by substrate thickness detection means, not shown. Thesubstrate thickness detection means is not explained in detail. Thedescription of the operating principle and the detailed mechanism isalso not made for simplicity.

With the optical pickup device 30 of the second embodiment, satisfactoryreproduction may be expected from the optical disc 5 having the discsubstrate thickness t₁=0.6 mm and the optical disc 7 having thesubstrate thickness t₂=1.2 mm.

The third embodiment is explained by referring to FIG. 15. The presentthird embodiment is similarly directed to an optical pickup device 41configured for reproducing information signals from an optical disc 37with a substrate thickness of 0.6 mm and an optical disc 38 with asubstrate thickness of 1.2 mm. The optical pickup device includes alaser diode 31 for radiating the playback laser light to the opticaldiscs 37 and 38 and an objective lens 36 for condensing the playbacklaser light from the laser diode 31 on the signal recording surfaces(information surfaces) of the optical discs 37 and 38. The opticalpickup device also includes an aperture varying mechanism 35 for settingthe numerical aperture NA of the objective lens 36 for the optical disc37 with the substrate thickness of 0.6 mm and the numerical apertureNA_(o) for the optical disc 38 with the substrate thickness of 1.2 mm to0.6 and to (0.45/0.78)×λ_(o), respectively, where λ_(o) denotes anumerical value equal to the wavelength of the laser light radiated fromthe laser diode 31 in μm, and a photodetector 40 for detecting thereturn light from the optical discs 37, 38. The aperture varyingmechanism 35 is arranged between the objective lens 36 and the laserdiode 31.

With the optical pickup device 41, the laser light radiated from thelaser diode 31 is collimated by a collimator lens 32 and split by adiffraction lattice 33 into three beams, that is a 0'th order light beamand ±1st order light beams. These three beams are conducted via a beamsplitter 34 and an aperture varying mechanism 35 to an objective lens 36so as to be condensed thereby on the recording surfaces of the opticaldiscs 37, 38. The return light from the signal recording surfaces of theoptical discs 37, 38 falls on the beam splitter 34 via the objectivelens 36 and the aperture varying mechanism 35. The beam splitter 34reflects part of the return light by a reflective surface 34R towards amulti-lens 39 which causes the return light to fall on the photodetector40. The photodetector converts the light volume of the return light intoelectrical signals. Thus, with the present optical pickup device 41,servo signals and RF signals are produced. Meanwhile, the objective lens36 of the optical pickup device 41 of the present third embodiment is aninfinite multiplication factor lens.

The laser light from the laser diode 31 transmitted from the beamsplitter 34 falls on the objective lens through the aperture varyingmechanism 35, as described above.

The aperture varying mechanism 35 arranged between the laser objectivelens 36 and the laser diode 31 is such an aperture varying means capableof changing the size of the aperture depending upon the substratethickness of the optical discs 37 and 38 and consequently changing thenumerical aperture of the objective lens 36.

Similarly to the optical pickup device of the first and secondembodiments, the optical pickup device 41 is capable of discriminatingthe difference in the substrate thickness of the optical disc 37 withthe substrate thickness of 0.6 mm and the optical disc 38 with thesubstrate thickness of 1.2 mm by substrate thickness detection means notshown. The substrate thickness detection means is not explained indetail. The description of the operating principle and the detailedmechanism of the aperture varying mechanism 35 of the optical pickupdevice 41 has been made above and hence is not made for simplicity.

With the optical pickup device 41 of the third embodiment, satisfactoryreproduction may be expected from the optical disc 37 having the discsubstrate thickness t₁=0.6 mm and the optical disc 38 having the discsubstrate thickness t₂=1.2 mm.

The fourth embodiment is explained by referring to FIG. 16. The presentfourth embodiment is similarly directed to an optical pickup device 42configured for reproducing information signals from the optical disc 37with a substrate thickness of 0.6 mm and the optical disc 38 with asubstrate thickness of 1.2 mm. The optical pickup device includes alaser diode 31 for radiating the playback laser light to the opticaldiscs and an objective lens 36 for condensing the playback laser lightfrom the laser diode 31 on the signal recording surfaces (informationsurfaces) of the optical discs 37 and 38. The optical pickup device alsoincludes an aperture varying mechanism 35 for setting the numericalaperture NA of the objective lens 36 for the optical disc 37 with thesubstrate thickness of 0.6 mm and the numerical aperture NA_(o) for theoptical disc 38 with the substrate thickness of 1.2 mm to 0.6 and to(0.45/0.78)×λ_(o), respectively, where λ_(o) denotes a numerical valueequal to the wavelength of the laser light radiated from the laser diode31 in μm, and a photodetector 40 for detecting the return light from theoptical discs 37, 38. The aperture varying mechanism 35 is arrangedbetween the objective lens 36 and the laser diode 31.

With the optical pickup device 42, the laser light radiated from thelaser diode 31 is collimated by a collimator lens 32 and split by adiffraction lattice 33 into three beams, that is a 0'th order light beamand ±1st order light beams. These three beams are conducted via a beamsplitter 34 and an aperture varying mechanism 35 to an objective lens 36so as to be condensed thereby on the recording surfaces of the opticaldiscs 37, 38. The return light from the signal recording surfaces of theoptical discs 37, 38 falls on the beam splitter 34 via the objectivelens 36. The beam splitter 34 reflects part of the return light by areflective surface 34R towards a multi-lens 39 which causes the returnlight to fall on the photodetector 40. The photodetector converts thelight volume of the return light into electrical signals. Thus, with thepresent optical pickup device 42, servo signals and RF signals areproduced. Meanwhile, the objective lens 36 of the optical pickup device42 of the present first embodiment is of a simplified construction of aninfinite multiplication factor objective lens.

The laser light from the laser diode 31 is passed through the collimatorlens 32, diffraction lattice 33 and the aperture varying mechanism 35 inthis order so as to fall on the objective lens 36 via the beam splitter34, as described above.

The aperture varying mechanism 35 arranged between the diffractionlattice 33 and the beam splitter 34 is such an aperture varying meanscapable of changing the size of the aperture depending upon thesubstrate thickness of the optical discs 37 and 38 and consequentlychanging the numerical aperture of the objective lens 36.

Similarly to the optical pickup devices of the first to thirdembodiments, the optical pickup device 42 is capable of discriminatingthe difference in the substrate thickness of the optical disc with thesubstrate thickness of 0.6 mm and the optical disc with the substratethickness of 1.2 mm by substrate thickness detection means, not shown.The substrate thickness detection means is not explained in detail. Thedescription of the operating principle and the detailed mechanism of theoptical pickup device 42 has been made above and hence is not made forsimplicity.

With the optical pickup device 42 of the fourth embodiment, satisfactoryreproduction may be expected from the optical disc 37 having the discsubstrate thickness t₁=0.6 mm and the optical disc 38 having the discsubstrate thickness t₂=1.2 mm.

A fifth embodiment is explained by referring to FIG. 17. The presentfifth embodiment is similarly directed to an optical pickup device 44configured for reproducing information signals from an optical disc 37with a substrate thickness of 0.6 mm and an optical disc 38 with asubstrate thickness of 1.2 mm. The optical pickup device includes alaser diode 31 for radiating the playback laser light to the opticaldiscs 37 and 38 and an objective lens 36 for condensing the playbacklaser light from the laser diode 31 on the signal recording surfaces(information surfaces) of the optical discs 37 and 38. The opticalpickup device also includes an aperture varying mechanism 43 for settingthe numerical aperture NA of the objective lens 36 for the optical disc37 with the substrate thickness of 0.6 mm and the numerical apertureNA_(o) of the objective lens 36 for the optical disc 38 with thesubstrate thickness of 1.2 mm to 0.6 and to (0.45/0.78)×λ_(o),respectively, where λ_(o) denotes a numerical value equal to thewavelength of the laser light radiated from the laser diode 31 in μm,and a photodetector 40 for detecting the return light from the opticaldiscs 37, 38.

With the present fifth embodiment, the aperture varying mechanism 43 isarranged between the objective lens 36 and the laser diode 31,specifically, between the collimator lens 32 and the diffraction lattice33. In addition, the aperture varying mechanism is configured for beingintroduced into and extracted from the opening portion of the collimatorlens 32.

With the optical pickup device 44, the laser light radiated from thelaser diode 31 is collimated by the collimator lens 32 and transmittedthrough the aperture varying mechanism 43 so as to be split by adiffraction lattice 33 into three beams, that is a 0'th order light beamand ±1st order light beams. These three beams are conducted via a beamsplitter 34 to the objective lens 36 so as to be condensed thereby onthe recording surfaces of the optical discs 37, 38. The return lightfrom the signal recording surfaces of the optical discs 37, 38 falls onthe beam splitter 34 via the objective lens 36. The beam splitter 34reflects part of the return light by a reflective surface 34R towards amulti-lens 39 which causes the return light to fall on the photodetector40. The photodetector converts the light volume of the return light intoelectrical signals. Thus, with the present optical pickup device 44,servo signals and RF signals are produced. Meanwhile, the objective lens36 of the optical pickup device 44 is an infinite multiplication factorobjective lens.

The laser light from the laser diode 31 is transmitted through thecollimator lens 32, the aperture varying mechanism 43 that can beintroduced into and extracted from the opening portion of the objectivecollimator lens 32 and the diffraction lattice 33, in this order, beforebeing incident on the objective lens 36 via the beam splitter 34.

The aperture varying mechanism 43 arranged between the collimator lens32 and the diffraction lattice 33 is such an aperture varying meanscapable of changing the size of the aperture depending upon thesubstrate thickness of the optical discs and consequently changing thenumerical aperture of the objective lens 36.

Similarly to the optical pickup devices of the first to thirdembodiments, the optical pickup device 44 is capable of discriminatingthe difference in the substrate thickness of the optical disc with thesubstrate thickness of 0.6 mm and the optical disc with the substratethickness of 1.2 mm by substrate thickness detection means, which is notshown and is not explained in detail for simplicity.

With the optical pickup device 44 of the fifth embodiment, satisfactoryreproduction may be expected from the optical disc 37 having the discsubstrate thickness t₁=0.6 mm and the optical disc 38 having the discsubstrate thickness t₂=1.2 mm.

The optical pickup device according to the present invention is notlimited to the above-described embodiments. Thus the optical pickupdevice according to the present invention may be employed forreproducing an optical disc having a substrate thickness of 0.8 mm andan optical disc having a substrate thickness of 1.2 mm. Such opticalpickup device is explained by referring to FIGS. 18 and 19. The overallconstruction is similar to that shown in FIG. 2 and hence is notexplained specifically.

The present sixth embodiment is directed to an optical pickup devicecapable of reproducing information signals from an optical disc 9 havinga substrate thickness t₁ of 0.8 mm as shown in FIG. 18 and an opticaldisc 11 having a substrate thickness t₂ of 1.2 mm as shown in FIG. 19.

For reproducing information signals from the optical disc 9 with asubstrate thickness t₁ of 0.8 mm, the light stop of the aperture varyingmechanism 12 and the aperture diameter of the objective lens 10 are setto 4.32 mm and to 4.32, respectively, for setting the numerical apertureof the objective lens 10 to 0.6. On the other hand, for reproducinginformation signals from the optical disc 11 with a substrate thicknesst₂ of 1.2 mm, the light stop of the aperture varying mechanism 12 andthe aperture diameter of the objective lens 10 are set to 2.88 mm and to2.88, respectively, for setting the numerical aperture of the objectivelens 10 to 0.4. At this time, the laser light having the wavelength of680 nm is radiated from the laser diode so that the focal length will be3.6 mm.

Evaluation of the operation of the optical pickup device of the sixthembodiment is done with the Marechal criterion of 0.07 rmsλ as areference.

The objective lens 10 has the following lens dimensions. The lensdimensions are given in the order of the radius of curvature, spacingand the refractive index according to respective planes. The spacingsand the radius of curvature of the object planes are both infinite (∞).

The radius of curvature, spacing and the refractive index of the firstplane are 2.4082, 2.460000 and 1.585352, respectively. The non-sphericalcoefficients of the first plane shown in the equation (5) arek=−0.153983, A=−0.474540E−03, B=0.0000000E−04, C=0.000000E+00 andD=0.000000E+00.

The radius of curvature and the spacing of the second plane are−10.48584 and 2.03311, respectively. The non-spherical coefficients ofthe second plane shown in the equation (5) are k=−0.17298E09,A=0.16670284E−01, B=0.000000E+00, C=0.000000E+00 and D=0.000000E+00.

As to the lens design specifications, the numerical aperture NA is0.60000, the wavelength is 680.00 nm, the focal lengths is 3.6, and animage height is 0.05000 mm, for the substrate thickness t₁=0.8 mm,whereas, for the substrate thickness t₁=1.2 mm, the numerical apertureNA is 0.40000, the wavelength is 680.00 nm, the focal length is 3.6, andan image height is 0.05000 mm.

The on-axis wavefront aberration is shown in FIGS. 20A, 20B, 21A and21B. FIG. 20A shows changes in the aberration in the meridinal imagesurface with respect to changes in the diameter of the pupil of theobjective lens 10 for the disc substrate thickness t₁=0.8 mm and thenumerical aperture of the objective lens of 0.6. FIG. 20B shows changesin the aberration in the sagittal image surface with respect to changesin diameter of the pupil of the objective lens 10. FIG. 21A showschanges in the aberration in the meridinal image surface with respect tochanges in the diameter of the pupil of the objective lens 10 for thedisc substrate thickness t₂=1.2 mm and the numerical aperture of theobjective lens of 0.4. FIG. 21B shows changes in the aberration in thesagittal image surface with respect to changes in diameter of the pupilof the objective lens 20. The on-axis aberration of the optical systemis 0.023 rmsλ and 0.029 rmsλ for the disc substrate thickness t₁=0.8 mmand the numerical aperture of the objective lens of 0.6 and for the discsubstrate thickness t₂=1.2 mm and the numerical aperture of theobjective lens of 0.4, respectively. In both of these cases, theMarechal criterion was not higher than 0.07 rmsλ. Thus, with the sixthembodiment, high-quality reproduction may be achieved with the two sortsof the optical discs, that is the disc 9 with the disc substratethickness t₁=0.6 mm and the optical disc 11 with the disc substratethickness t₂=1.2 mm.

The aperture varying mechanism employed in the above-described first tosixth embodiments may be configured as shown in FIGS. 22A, 22B, 23 and24, instead of being configured as shown in FIG. 5.

The aperture varying mechanism 13 shown in FIGS. 22A and 22B varies theaperture size using a liquid crystal shutter portion 14. FIG. 22Arepresents the state in which the liquid crystal shutter portion 14 isclosed for realizing a small-sized aperture 15. FIG. 22B represents thestate in which the liquid crystal shutter portion 14 is opened forrealizing a large-sized aperture 16.

An aperture varying mechanism 17 shown in FIG. 23 varies the aperturesize by changing over the aperture portions by moving the discinherently formed with a small aperture 18 and a large aperture 19 inthe direction shown by arrow L.

An aperture varying mechanism 50 shown in FIG. 24 varies the aperturesize by changing over the aperture portions by rotating the discinherently formed with a small aperture 51 and a large aperture 52 inthe direction shown by arrow R.

The aperture varying means for varying the size of the aperture of theobjective lens depending upon the thickness of the optical recordingmedium may be arranged between the objective lens and the light source(e.g., as shown in FIG. 4), or between the objective lens and theoptical recording medium, as shown in FIG. 25.

The aperture varying mechanism may be provided on the surfaces or withinthe inside of the objective lenses 4 and 10. For example, FIG. 26 showsan optical pickup device having an aperture varying mechanism arrangedwithin the objective lens 4, and FIG. 27 shows an optical pickup devicehaving an aperture varying mechanism arranged on a surface of theobjective lens 4.

The optical pickup device according to the present invention may beapplied in recording information signals on the optical disc.

What is claimed is:
 1. An optical pickup device capable of recording and/or reproducing at least two different sorts of optical recording media, comprising: a light source radiating a light beam; an objective lens for condensing the light from said light source; substrate thickness detection means for detecting a thickness of a substrate of the optical recording medium; aperture varying means for varying the size of an aperture of said objective lens depending upon an output signal from said substrate thickness detection means; and light detection means for detecting the return light from said optical recording medium.
 2. The optical pickup device as claimed in claim 1, wherein said optical recording media are an optical recording medium having a substrate thickness of 0.6 mm and an optical recording medium having a substrate thickness of 1.2 mm and wherein said aperture varying means sets a numerical aperture NA of said objective lens with respect to the optical recording medium having a substrate thickness of 0.6 mm to 0.6, while setting a numerical aperture NA_(o) of said objective lens with respect to the optical recording medium having a substrate thickness of 1.2 mm to: NA_(o)≦(0.45/0.78)×λ_(o) where NA_(o) is a numerical aperture of said objective lens with respect to said optical recording medium having the substrate thickness of 1.2 mm and λ_(o) is a numerical value equal to the wavelength in μm of light radiated from said light source.
 3. The optical pickup device as claimed in claim 1 wherein said aperture varying means is arranged between said objective lens and said light source.
 4. The optical pickup device as claimed in claim 1 wherein said aperture varying means is arranged between said objective lens and said optical recording medium.
 5. The optical pickup device as claimed in claim 1 wherein said aperture varying means is arranged on the surface of said objective lens.
 6. The optical pickup device as claimed in claim 1 wherein said aperture varying means is arranged within the inside of said objective lens.
 7. The optical pickup device as claimed in claim 1 wherein said aperture varying means varies the aperture size by rotating respective blades of a light stop mechanism having a plurality of blades.
 8. The optical pickup device as claimed in claim 1 wherein said aperture varying means varies the aperture size using a liquid crystal shutter.
 9. The optical pickup device as claimed in claim 1 wherein said aperture varying means varies the aperture size by moving a plate having plural openings for changing over said openings.
 10. An optical pickup device capable of recording and/or reproducing optical recording media, comprising: a light source radiating a light beam; an objective lens for condensing the light from said light source; a liquid crystal shutter arranged in an optical path between the light source and said objective lens, which is operative to electrically change at least a size of a light beam passing said liquid crystal shutter; a substrate thickness detection means for detecting a substrate thickness of the optical recording media which is loaded; and liquid crystal control means for controlling said liquid control shutter to minimize the wave front aberration of a beam spot converged onto said optical recording media in accordance with the result of the substrate thickness detection means.
 11. The optical pickup device as claimed in claim 10, further comprising light detection means for detecting a return light from said optical recording media, and said substrate thickness detection means detects a difference in substrate thickness using RF or servo signals detected by said light detection means.
 12. The optical pickup device as claimed in claim 10, wherein the substrate thickness detection means detects a difference in substrate thickness using information regarding substrate thickness recorded on said optical recording media.
 13. A wave front aberration compensating apparatus for an optical data recording and reproducing apparatus to correct a wavefront aberration of light emitted from a light source and converged onto a data recording medium through an image forming optical system, said apparatus comprising: a liquid crystal shutter arranged in an optical path between the light source and said data recording medium, which is operative to electrically change at least a size of a light beam passing said liquid crystal shutter; a substrate thickness detection means for detecting a substrate thickness of the data recording medium which is loaded; and liquid crystal control means for controlling the liquid crystal shutter to minimize the wave front aberration of a beam spot converged onto said data recording medium in accordance with the result of the substrate thickness detection means.
 14. The wave front aberration compensating apparatus as claimed in claim 13, further comprising light detection means for detecting a return light from said data recording medium, and said substrate thickness detection means detects a difference in substrate thickness using RF or servo signals detected by the light detection means.
 15. The wave front aberration compensating apparatus as claimed in claim 13, wherein the substrate thickness detection means detects the difference in substrate thickness using information regarding substrate thickness recorded on the data recording medium. 